1. Field of the Invention
This invention relates to a method for detecting a step and a phase change of an object to be observed from the image information of the object obtained by an interference microscope such as a differential interference contrast microscope (DIC microscope), and an optical apparatus using the method.
2. Description of Related Art
A microscope utilizing interference, such as a DIC microscope, has been widely used to observe the microscopic structure of a human body or an IC pattern because information on a phase change and a step of an object can be visualized by interference. In particular, various attempts have recently been made to use the DIC microscope for the inspection of minute projections (bumps) for intimate contact prevention of a magnetic head, provided on the surface of a magnetic disk, the measurements of a defect in a phase-shift reticle used for pattern exposure of a semiconductor and the amount of phase difference, and the positioning device of a semiconductor wafer.
For example, each of Japanese Patent Preliminary Publication Nos. Hei 5-149719 and Hei 7-248261 discloses a technique of applying a DIC microscope, which is thought of as a shearing interferometer or a Mach-Zehnder interferometer, to the detection of a defect in a phase-shift reticle and the measurement of phase. Japanese Patent Preliminary Publication No. Hei 7-239212 discloses a technique that the DIC microscope is used to detect the edge of a positioning mark provided on a semiconductor wafer, thereby positioning the semiconductor wafer.
In each of these techniques, however, conventional interference measuring technology is merely applied to the DIC microscope, and the influence of diffraction of light on the surface of an object is not taken into account. Moreover, the influence of a change in intensity of light caused by a change in reflectance or transmittance of light on the object is also not taken into account.
For these influences of the diffraction and intensity change of light on the object, in Japanese Patent Preliminary Publication No. Hei 9-15504, the present inventor clarifies the imaging characteristics of the DIC microscope and provides an approach that extracts the phase information of the object from an image obtained by the DIC microscope.
The DIC microscope is such that a phase change on the surface of the object is converted into an image intensity distribution. Conversely, it is conceivable that the intensity distribution of a differential interference contrast image is analyzed and thereby the phase change on the surface of the object can be detected. Further, it is set forth in Hei 7-239212 that since the edge of the step of the object brings about an abrupt phase change and as a result, the image intensity distribution is also abruptly changed, a portion of the image intensity distribution abruptly changed is extracted from the differential interference contrast image, and thereby the position of the step of the object can be detected.
Japanese Patent Preliminary Publication No. Hei 5-256795 discloses a technique that uses the differential interference contrast image of a normal sample as a reference image and compares this reference image with the image of the object, thereby detecting foreign matter contained in the object.
A Michelson type or Mirau type interference microscope is also used to measure the phase distribution of the object.
In the case where the phase distribution of the object is derived from the intensity distribution of the differential interference contrast image, the phase change of the object cannot be detected with accuracy if the object contains factors other than the phase change of the object caused by a change in transmittance or reflectance of light or a change in intensity of illumination light in the object.
For the measurement of a change in the amount of phase due to the step of the object, when the height of the step is relatively small, a fringe scanning technique employed for interference measurement is combined with the operation of the DIC microscope and thereby the phase information of the object can be extracted. In the fringe scanning technique, four images in which the amounts of retardation between polarized components are different must be formed for calculation, and thus the problem of reducing a processing time cannot be solved.
When the amounts of retardation between polarized components are 0 and xcfx80, image intensity distributions differ materially. Thus, in order to detect a correct amount of phase, an image sensor whose dynamic range is wide becomes necessary, which makes an optical apparatus complicated.
When the height of the step of the object increases, the problem arises that the amount of phase measured by the combination with the fringe scanning technique becomes smaller than in an actual step.
Even when the Michelson type or Mirau type interference microscope is used to measure the phase distribution of the object, there is the problem that the object, which has the step, is affected by diffraction and scattering of light at the step and a correct amount of phase cannot be measured.
It is, therefore, an object of the present invention to provide a method for detecting the height of a step and the amount of phase change of an object in a shorter time than in a conventional way and making a precise measurement when the height of the step increases, by separating phase information and intensity information from an interference image of the object obtained by an interference microscope, and a detection apparatus (optical apparatus) using this method.
In order to achieve the above object, the detection apparatus according to the present invention includes a microscope optical system having a light source, an illumination optical system for leading light emitted from the light source to an object, an imaging optical system for forming the image of the object, and at least one polarizing member for separating the light from the light source into two polarized components; an adjusting member for changing the amount of retardation between the two polarized components; and an image pickup member for photographing a differential interference contrast image of the object. The detection apparatus, operated through a process for photographing two differential interference contrast images relative to the object in which the amounts of retardation are equal, but have different signs, is provided with a processing unit operated through a calculation process for performing a differential calculation and a s ed calculation relative to respective pixels corresponding to the two differential interference contrast images to obtain differential image information and summed image information and another calculation process for detecting the amount of phase on the surface of the object by using one of the following equations:
"PHgr"(x,y)=kxc2x7{(1xe2x88x92cos xcex8)xc2x7d(x,y)/xcex1}/{sin xcex8xc2x7[1xe2x88x92{d(x,y)/xcex1}2/2]}
"PHgr"(x,y)=kxc2x7tanxe2x88x921[{(1xe2x88x92cos xcex8)xc2x7d(x,y)/xcex1}/{sin xcex8xc2x7[1xe2x88x92{d(x,y)/xcex1}2/2]}]
where xcex8 is the amount of retardation; "PHgr"(x,y) is the amount of phase on the surface of the object corresponding to the differential image information and the summed image information; when the differential image information is represented by D(x,y), d(x,y) is image information in which the differential image information D(x,y) is deconvoluted by using the optical transfer function of the microscope optical system; when the summed image information is represented by S(x,y), xcex1 is the average value of the summed image information S(x,y); and k=xcex/4xcfx80; where xcex is a wavelength.
The detection apparatus according to the present invention includes an interference optical system having a light source, an illumination optical path for leading light emitted from the light source to an object, and a reference optical path for leading the light from the light source to a reference surface; and an image pick-up member for photographing an interference image of the object formed by the interference optical system. In this case, the detection apparatus is provided with a processing unit operated through a calculation process for detecting the amount of phase on the surface of the object by using one of the following equations:
"PHgr"(x,y)=kxc2x7{h(x,y)/Jm(x,y)}/{[1xe2x88x92{h(x,y)/Jm(x,y)}2/2]}
"PHgr"(x,y)=kxc2x7tanxe2x88x921[kxc2x7{h(x,y)/Jm(x,y)}/{[1xe2x88x92{h(x,y)/Jm(x,y)}2/2]}]
where when image information in which the phase distribution of the object is picturized is represented by H(x,y), h(x,y) is image information in which the image information H(x,y) is deconvoluted by using the optical transfer function of the interference optical system; when image information in which an intensity distribution is picturized is represented by J(x,y), Jm(x,y) is image information in which the maxima of the image information J(x,y) are enveloped; "PHgr"(x,y) is the amount of phase on the surface of the object; and k=xcex/4xcfx80, where xcex is a wavelength.